[Manifest] URI IS "https://api.facepunch.com/api/public/manifest/?public_key=j0VF6sNnzn9rwt9qTZtI02zTYK8PRdN1"
I saw this in my RCON and don't know what it means

It is for developers that want to use the JSON code (the stuff you see in the link) in their apps.
Like i use it in my application to alert me when a new rust server update is available so i can update my servers immediately.
JSON Introduction
This is mostly for developers so you don't need to worry about it.